Recently, circuit boards have been changed from single-face boards having circuits formed on one face, to double-face boards and multilayer circuit boards so as to achieve compact and high-performance apparatuses. Many of the boards adopted at present are advanced multilayered ones. In the meantime, a printing technique using a filling head, i.e., a squeegee is employed in order to press and fill a paste into openings such as via holes or through holes in these circuit boards, or to press and fill the paste into openings of a metal mask to form circuits on the board. A lot of conditions are involved in the printing technique, for instance, hardness of the squeegee, angle of the squeegee to a printing face, pressure of the squeegee to the printing face, filling speed of the squeegee to the openings of the printing face, etc. Even a slight error in setting of the conditions adversely influences the quality of printing films, and therefore the set conditions are adjusted based on experiences on the production line, thereby improving yields in the paste filling process and quality of products.
Some adaptations are also made to the aforementioned squeegee in the filling process. For example, the squeegee is improved in shape to have a flat, a sharp, or a curved contact face as the printing face. In one arrangement, a heater is built in the squeegee to heat the squeegee itself to a fixed temperature, thereby lowering a viscosity of the paste and increasing filling efficiency for the paste to the openings of the printing face.
The viscosity of the paste increases in proportion to an operating time of the squeegee. For stabilizing the viscosity, therefore, the paste is exchanged at a fixed time interval, or an organic solvent is added to the paste to adjust the viscosity.
Meanwhile, the amount of the paste filled into the openings, e.g., via holes and through holes, etc. or the openings of the metal mask is controlled and stabilized by filling the paste up to a surface of the openings.
An example of the conventional circuit board-manufacturing method will be described with reference to FIGS. 13A, 13B.
FIGS. 13A, 13B respectively represent examples of a manufacturing method and a manufacturing apparatus for circuit boards in the filling process according to the conventional art.
In FIGS. 13A, 13B, 300 denotes a metal mask placed on a base material 301. 303 and 305 denote squeegees which move on the metal mask 300 while keeping touch with the metal mask 300. 304 denotes a paste, and 302 denotes a stage. The base material 301 is disposed in touch with on the stage 302. The metal mask 300 is arranged with a slight space above the base material 301, onto which the paste 304 is supplied. The squeegees 303, 305 located above the metal mask 300 can move in an up/down direction and in a printing direction of the squeegees 303, 305.
The paste is filled in the thus-constructed apparatus in a process described herein. The base material 301 is first set at a predetermined position on the stage 302. A through hole 301a formed in the base material 301 is registered with a hole 300a formed in the metal mask 300, and the metal mask 300 is set above the base material 301. The paste 304 is supplied onto the metal mask 300. The squeegees 303, 305 are lowered to hold the base material 301 by pressing a predetermined pressure on to the material 301. Thereafter, the squeegees 303, 305 are moved in the printing direction to move the paste 304 on the metal mask 300, thereby printing onto the base material 301. The paste 304 is pressed and filled in the through holes 301a formed in the base material 301 in this manner.
In the above paste filling process in the conventional art, a filling shape or the filling amount of the paste 304 is greatly changed even by mechanical errors, resulting in blanking, insufficient printing, or similar filling failures. When the flat squeegee 305 shown in FIG. 13B is used, a contact angle of the squeegee to the printing face of the metal mask 300 or base material 301 is hard to fix and maintain. The amount of the paste filled in the through hole 301a varies greatly and is considerably difficult to control properly. This is an important issue. In the case of the square squeegee 303 as shown in FIG. 13A, the contact angle of the squeegee to the printing face of the metal mask 300 or base material 301 is easy to set. A front end part of the square squeegee 303 in touch with the printing face of the base material 301 is not sufficiently sharp, and consequently the paste 304 is left on the metal mask 300 or base material 301. The front end part of the square squeegee can hold only a small amount of the paste 304. This makes it necessary to frequently replenish the paste 304, and thus deteriorates productivity. When a viscosity of the paste 304 is increased because of the evaporation of volatile components or when the paste of a high viscosity is used, considerably less paste is filled than if kept under the originally properly set conditions. In other words, the filling condition should be re-set during operation although this takes a long time. This is another issue. In the case where a heater is incorporated in the squeegee 303, 305, this arrangement merely lowers the viscosity of the paste 304. This does not effectively influence the rolling or filling of the paste 304 and raises the necessity for setting and adjusting the conditions again. Poor productivity still remains as an issue.
The object of the present invention is therefore to provide a printing method and a printing apparatus whereby a sufficient filling amount of a printing material into openings of a body to be printed is secured in conformity with various filling conditions, to thereby improve yields, quality, and productivity for printing with the printing material.